With the desire to utilize “green” energy and renewable energy sources, there is a desire to incorporate these intermittent types of energy sources into the power grid. Intermittent sources include, but are not limited to wind, solar, photovoltaics and wave power. For example, if winds are not present, the wind turbines do not generate electrical energy; however, when they do produce energy, the current electrical grid cannot always handle large quantities of the energy produced. Grid connected energy storage devices would enable capturing excess energy from these intermittent renewable energy sources, and releasing the stored energy on the grid when it is needed. This combination of renewable energy sources and grid energy storage would support energy independence, reduced emissions and renewable energy sources. There is also a need for reliable grid energy storage devices so as to relieve transmission congestion, allow for energy price arbitrage, and improve the overall quality of the power grid.
It is believed that flow batteries are a viable solution for creating and improving grid storage. Flow batteries can potentially provide efficient modular energy storage while providing a low cost. They can be independently operated and provide adequate energy and power ratings by utilizing replenishable-liquid reactants and have low cycling impacts and a long life. Flow batteries also have other uses as it relates to microgrid or small power systems and for use as backup power supplies. However, the cost of these systems has prevented wide-scale deployment. A major portion of the system cost is in the flow battery cell stack and the associated anolyte and catholyte. To a large extent, the stack costs are limited by the current density that can be put through the cell stack. A higher current density enables more power to be generated in a given cell stack and effectively decreases the cost per watt. But with current state of art low surface area electrode, higher current density will lead to higher energy loss which increases operational cost. Thus, the electrodes need to have a much greater electroactive surface area, while still managing to minimize cost. Current flow battery systems use carbon-based materials, such as carbon felts, for the electrodes.
Referring to FIG. 1, it can be seen that a known flow battery configuration is designated generally by the numeral 10. The battery 10 is provided in a single cell configuration but skilled artisans will appreciate that multiple cells can be incorporated into a stack, and multiple stacks can be employed. In any event, a flow battery comprises an anode 12 and a cathode 14, both of which are referred to as electrodes. An anolyte tank 16 and a catholyte tank 18 direct respective fluid materials through an anode flow area 22 and a cathode flow area 24. A separator membrane 20 is used to separate the anolyte flow area 22 from the catholyte flow area 24 while allowing ion exchange between the two flow areas. As these materials flow through their respective channels, electrical power is generated by redox reactions, in which electrons are drawn through an external electric load 26 as schematically represented by a light bulb.
Skilled artisans will appreciate that the flow battery is a rechargeable battery in which anolyte and catholyte containing one or more dissolved electroactive species flows through the electrochemical cell that converts the chemical energy directly into electricity. Flow batteries can be recharged by re-flowing the electrolyte liquids through the flow areas as an external electrical power source is applied to the electrodes, effectively reversing the electrical generation reactions. The flow battery is advantageous in that the reaction of active species in the electrolyte permits external storage of reactants, thereby allowing independent scale up of power and energy density specifications. Moreover, the ability to externally store the reactants avoids self-discharge that is observed in other primary and secondary battery systems. As such, the energy is effectively stored in the anolyte and catholyte tanks until needed by the load.
Various chemistries are utilized in the operation of flow cell batteries. In particular, different types of anolyte and catholyte materials may be utilized. For example, the zinc bromine system may be utilized wherein zinc plating is maintained in the cell stack. These types of configurations utilize high efficiency and low cost reactants. Vanadium redox technology may also be utilized. This provides high efficiency but low energy density. There is minimal risk of cross-contamination between the materials, however the vanadium is an expensive material and the pentoxide utilized is considered a hazard after it is no longer usable. Another type of flow battery utilizes iron-chrome. Its advantage is in the use of low cost reactants, however it currently only provides for smaller type systems in comparison to the zinc-bromine or vanadium redox embodiments. There are additional chemical couples that could be utilized in a flow battery configuration.
Flow batteries typically use carbon felt electrodes. This kind of configuration is advantageous in that the carbon electrodes are chemically compatible with the typical anolyte and catholyte solutions and provide relatively high surface area and good electrical conductivity. The carbon felt provides for a high number of reaction sites and is a discrete component that is sandwiched or disposed between the bipolar plates, which are typically a solid carbon or conductive polymer material, and the membrane separator. The carbon felts are directly in contact with the bipolar plate. Other materials that can be used for the electrodes are carbon or graphite particles that are embedded directly into the bipolar plates. The significant drawback of the carbon felt electrodes is that it limits the desired current density. In particular, the current densities are believed to be limited by the lack of surface area and the density of electroactive reaction sites.
Therefore, there is a need in the art for flow batteries which utilize electrodes that have improved surface areas so as to allow for a higher density of reaction sites and, thus, the ability to store and generate higher power output. There is also the need to provide such an improved electrode that minimizes system level cost.